The FA68XX alloy family is a reliable and proven fin alloy family, characterized by a high post-braze strength, and good sagging resistance during brazing. The alloy is designed to form large grains during brazing, which is beneficial for the sagging behavior and hinders braze metal penetration into the core. The high manganese and silicon content contribute to a higher strength after brazing. The corrosion resistance of the fin alloys is good, with a low self corrosion rate. FA68xx is available in four different zinc levels to allow for a tailored cathodic protection of tubes and fillets.
Post braze grain structure. Clad product in the left upper corner and the unclad product in the right lower corner.
The sagged distance is measured in a CAB batch furnace using a braze cycle with 20 minutes to 600 °C and 3 minutes dwell time. The sagging value is strongly influenced by both the gauge of the material and whether the sample is clad or not. The beam length is varied between 25 and 60 mm to accommodate for this.
Delivery condition temper and properties
Typical properties after brazing
* Measured on 70 μm material, 35 mm beam.
** Calculated from electric resistivity data measured at room temperature.
An important aspect of fins is that they should provide a cathodic protection of the tubes, due to a suitable potential difference between the tube and the fin. However, even though the fin has a low potential it should not corrode too fast, i.e. its self-corrosion rate should be low. Note in the picture that even after such an extreme exposure as 40 days of SWAAT, the tube and fillet are still very well protected by the fin.
FA6815 fin and FA7873 tube after 40 days SWAAT (ASTM G85). No perforations, only superficial corrosion on the tubes and no or minor fillet corrosion.
The corrosion potential (ASTM G69) was measured on brazed unclad fin material.
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